Now is the time to plan your fall-applied herbicides!

Sandra L. Wick, Crop Production Agent

Now that row crop harvest is progressing and some fall moisture has been received, it is time to start planning your fall herbicide applications to control winter annual broadleaf weeds and grasses ahead of corn, grain sorghum or soybeans.

Tillage is still a proven control for winter annuals, however, with the adoption of no-till systems, herbicides have proven to be an effective weed control if certain factors are kept in mind for the pesticide application including timing, the proper rates, herbicide selection and types of applications.   Fall applications during late October and through November can greatly assist control of difficult winter annuals and should be considered when performance of spring-applied pre-plant weed control has not been adequate according to Dr. Dallas Peterson, K-State Research and Extension, Weed Management Specialist.   The later fall application will allow coverage of the fall winter annual weeds and persist longer in the spring.  Henbit, marestail, mustards along with winter grasses, such as downy brome or cheat, frequently are some of the most troublesome weeds we try to manage with these fall herbicide applications.

Fall applications have another side-benefit. While it is always important to manage herbicide drift, herbicide applications made after fall frost have less potential for drift problems onto sensitive targets along with most weeds still actively growing which allows for thorough coverage that is critical for effective control.

There are several herbicide options for fall application. Peterson stresses that if residual weed control is desired for corn or grain sorghum, atrazine is among the lowest-priced herbicides. However, if atrazine is used, that will lock the grower into planting corn or sorghum the following spring, or leave the land fallow during the summer and come back to winter wheat in the fall. If atrazine is applied too early, warm weather and moisture will reduce the length of residual. November is often the best time for atrazine applications.  Studies in Kansas indicate that a fall application of 2,4-D LVE at 2 lbs./acre was more effective (80% control) than a similar rate of 2,4-D amine (49% control). Dicamba + 2,4-D amine at 0.25 + 0.75 lbs./acre and picloram at 0.125 lbs./acre were also effective (>90% control) on musk thistle treated in the fall.

Atrazine is labeled in Kansas for fall application over wheat stubble or after row crop harvest (corn or grain sorghum) any time before December 31, as long as the ground is not frozen. Consult the atrazine label to comply with maximum rate limits and precautionary statements when applying near wells or surface water. No more than 2.5 lbs./acre of atrazine can be applied in a calendar year on cropland.

One-half to two pounds (maximum) per acre of atrazine in the fall, tank-mixed with 2,4-D and/or dicamba or Glyphosate, can give good burn down of winter annual broadleaf weeds -- such as henbit, dandelion, prickly lettuce, pepperweed, field pansy, evening primrose, and marestail -- and small, non-tillered winter annual grasses. But it will have a fairly limited residual control, however, most of the winter annuals are actively growing at this time of application, and it does a pretty good job of keeping the fields clean before planting next spring.  Foliar activity of atrazine is enhanced with crop oil concentrate, which should be included in the tank-mix.

Soybeans, on the other hand, are a little more limited as far as herbicide options.  No atrazine can be applied, so you must be more careful with your herbicide selection.  Herbicides might include Valor, Authority or Classic or other pre-mixes labeled for soybeans. All these options do a good job of control of the winter annuals, so the more important issue is timing and getting the herbicide application done.

Atrazine residual should control germinating winter annual broadleaves and grasses. When higher rates of atrazine are used, there should be enough residual effect from the fall application to control early spring-germinating summer annual broadleaf weeds such as kochia, common lambsquarters, wild buckwheat, and Pennsylvania smartweed – unless the weed population is triazine-resistant. Even though kochia is not considered a winter annual weed, lately it has tended to germinate as early as February.  So a higher rate of Atrazine along with Dicamba generally does a pretty good job of management.

Marestail, a winter annual, that can also germinate in the spring and summer, is an increasing problem in Kansas that merits special attention. “Where corn or grain sorghum will be planted next spring, fall-applied atrazine plus 2,4-D or dicamba have effectively controlled marestail rosettes and should have enough residual activity to kill marestail as it germinates in the spring,” states Peterson.   Atrazine alone will not be nearly as effective post-emergence on marestail as the combination of atrazine plus 2,4-D or dicamba. Dicamba is generally more effective than 2,4-D for marestail control.  Sharpen can be very good on marestail, but should be tank-mixed with 2,4-D, dicamba, atrazine, or glyphosate to prevent regrowth.

Peterson points out that if the spring crop will be corn, other residual herbicide options include ALS herbicides such as Autumn Super or Basis Blend. ALS-resistant marestail will survive an Autumn Super or Basis Blend treatment if applied alone. For burn down, producers should mix in 2,4-D, dicamba, and/or glyphosate. Winter annual grasses can also be difficult to control with atrazine alone. Success depends on the stage of brome growth. For downy brome control, 2 lbs./acre of atrazine plus crop oil concentrate (COC) has given excellent control, whereas 1 lb./acre has given only fair control. Volunteer wheat and brome species that have tillered and have a secondary root system developing will likely not be controlled even with a 2-lb rate. Adding glyphosate to atrazine will ensure control of volunteer wheat, annual brome grasses, and other winter annual grassy weeds. Atrazine antagonizes glyphosate, so if the two are used together, a full rate of glyphosate (0.75 lb. ae/A) is recommended for good control. The tank-mix should include AMS as an adjuvant.

Where fall treatments control volunteer wheat, winter annuals, and early-emerging summer annuals, producers should then apply a pre-emerge grass-and-broadleaf herbicide with glyphosate or paraquat at corn or sorghum planting time to control newly emerged weeds. Soils will be warmer and easier to plant where winter weeds were controlled in fall.

K-State Research and Extension has an excellent (free of charge) publication, “2019 Chemical Weed Control” that provides an extensive guide for weed management for field crops, pastures, rangeland and non-cropland.  This is available online or at any of our Post Rock Extension District Offices.

If you have further questions on crop weed management, contact Sandra at any Post Rock Extension District Office in Beloit, Lincoln, Mankato, Osborne or Smith Center.

Post Rock Extension District of K-State Research and Extension serves Jewell, Lincoln, Mitchell, Osborne, and Smith counties. Sandra may be contacted at swick@ksu.edu  or by calling Smith Center, 282-6823, Beloit 738-3597, Lincoln 524-4432, Mankato 378-3174, or Osborne 346-2521.  Join us on Facebook at “Post Rock Extension” along with our blog site at “postrockextension.blogspot.com.  Also remember our website is www.postrock.ksu.edu and my twitter account is @PRDcrops.